Electrospray ionization (“ESI”) is an important technique for the analysis of biological materials contained in solution by mass spectrometry. See, e.g., Cole, R. B. Electrospray Ionization Mass Spectrometry: Fundamentals, Instrumentation & Applications; John Wiley and Sons, Inc.: New York, 1997. Electrospray ionization was developed in the late 1980s and was popularized by the work of Fenn. See, e.g., Fenn J B, Mann M, Meng C K, Wong S F & Whitehouse C M (1989), Electrospray ionization for mass-spectrometry of large biomolecules, Science 246, 64-71. Simplistically, electrospray ionization involves the use of electric fields to disperse a sample solution into charged droplets. Through subsequent evaporation of the droplets, analyte ions contained in the droplet are either field emitted from the droplet surface or the ions are desolvated resulting in gas phase analyte ions. The source of the liquid exposed to the electric field and to be dispersed is ideally one of small areal extent as the size of the electrospray emitter directly influences the size of droplets produced. Smaller droplets desolvate more rapidly and have fewer molecules present per droplet leading to greater ionization efficiencies. These ions can be characterized by a mass analyzer to determine the mass-to-charge ratio. Further analyte structural information can be obtained by employing tandem mass spectrometry techniques.
Separation of analytes prior to electrospray ionization is important for minimizing ionization suppression and MS spectral complexity. Microfluidic capillary electrophoresis with integrated electrospray ionization has been demonstrated as a fast and efficient method of coupling a liquid phase chemical separation with mass spectroscopy detection. See, e.g., Anal. Chem. 2008, 50, 6881-6887; and Anal. Chem. 2015, 87, 2264-2272. Conventional microfluidic methods that employ electrokinetic flow of sample into the separation channel are subject to injection bias and cannot effectively be used for some on-device sample focusing methods. Further, the injection of a well-defined band of sample into the separation channel of the microfluidic device can be important to achieving an efficient separation.